As much as I'm in favor of dettering driving for most people, very high gas prices will punish the poor more than the drivers. The higher gas prices result, first, in higher food prices. Also, the people who live in poor areas are usualy the ones with the least public transit access. Without a real transit infrastructure, raising gas taxes is more punishment than deterrant. If the idea is to make less fuel efficient vehicles less enconomical, then put the taxes on the purchase, and base the taxes on MPG. (Of course then they'd have to do real fuel testing rather than the bogus numbers the EPA Estimates provide). For every MPG in city driving it gets under 30, add $1000, for every MPG under 20, add $5k. Funnel that tax money into transit and see what happens.
Yes, this is an issue, but it would be pretty easy to get waivers for businesses and farmers who are using their vehicles to transport goods so it wouldn't really impact food or other material prices. It could also be possible to have a needs based system like food stamps or something where if you make under $x then you get a discount. Or you could just apply it in major metro areas which is where most of the wealth is concentrated.
Alternatively, the fed govt could give tax incentives to towns/cities that improve their PT systems and allow credits for people who use those systems.
Taxing the cars is a good idea but it wouldn't do anything for the hundreds of thousands of gas guzzlers that are already out there.